Apparatus for treating wood chips with electrons

ABSTRACT

Apparatus for irradiating wood chips with electrons includes electron generator with means to inject electrons into conduit through which chips are pneumatically conveyed.

United States Patent [19 Free [ APPARATUS FOR TREATING WOOD CHIPS WITHELECTRONS [75] Inventor: David Free, West Vancouver,

Canada [73] Assignee: Radiation Development Co., Ltd.,

1 Vancouver, British Columbia,

Canada [22] Filed: Nov. 9, 1972 21 1 Appl. N02: 305,073

Related US. Application Data [63] Continuation-in-part of Ser. No.93,060, Nov. 27,

1970, abandoned.

[52] U.S. Cl 250/400, 250/398, 250/435,

1 250/492 [51] Int. Cl. H0lj 37/30 [58 Field of Search 250/396, 398,400,

[ Dec. 18, 1973 [56] References Cited UNITED STATES PATENTS 2,680,8146/1954 Robinson 250/495 X 2,820,165 1/1958 Robinson.. 250/495 X2,907,704 10/1959 Trump I. 250/49.5 X

FOREIGN PATENTS OR APPLICATIONS 1,394,142 2/1965 France 250/495 PrimaryExaminer+-William F. Lindquist Att0rneyStephen W. Blore et al.

57 ABSTRACT Apparatus for irradiating wood chips with electrons includeselectron generator with means to inject electrons into conduit throughwhich chips are pneumatically conveyed.

'7 Claims, 3 Drawing Figures PATENIEDHEB 18 ms 3.780.305

DAVID FREE INVENTOR BY BUCKHORN, BLORE, KLARQUIST & SPARKMAN ATTORNEYSAPPARATUS FOR TREATING WOOD CHIPS WITH ELECTRONS RELATED APPLICATIQNS vThis application is'a continuation-in-part of application Ser. No.93.060, filed Nov. 27, l970, now abandoned.

BACKGROUND OF THE INVENTION are manufactured at other sites such as atsawmills from scrap pieces, and other residue material accumulating atthe mill. It is not generally possible to place the chips intoprocessing immediately as they are produced or received, and at mostpulp mills large storage piles of chips are maintained. During storage,chips usually undergo deterioration in one form or another. In someinstances chips develop fungi growth that causes discoloration'of thechips and the resulting pulp and pulp product, and other cellularbreakdowns occur that decrease the ultimate yield of pulp and thus paperor other products from the chips. Depending upon the length of storagethis can result in from to per cent loss by weight.

Efforts have been made todevise methods of sterilizing chips so as tokill bacteria and fungi which cause chip deterioration, but economicallyfeasible and practically acceptable systems of chip treatment have notheretofore been developed.

SUMMARY OF THE INVENTION in accordance with the subject invention, woodchips are pneumatically conveyed through a specially shaped conduitthrough which electrons are passed transversely so as to impinge uponthe chips as they move through the conduit. The chips are conveyedthrough the conduit under conditions assuring that essentially allsurfaces of the chips are exposed to the electrons to effect irradiationand thus sterilization of all surface and some subsurface portions. Bysterilizing the chips in such manner the sterilization can be carriedout as chips are conventionally pneumatically conveyed from the chipproducing mechanism or from their point of receipt to the chip storagepile, but assuring complete exposure of all surface portions to theelectrons in an economically feasible manner.

In addition to enhancing the storage life of the chips and minimizingbreakdown of the chips from bacteria, fungi, and like micro-organisma,it has been further dis covered that the electron bombardment of thechips, if carried out at certain levels, enables the processing, thatis, cooking time, of the chips to be substantially reduced and increasethe amount of pulp producible from a given quantity of chips withbetterment of its physical properties.

DRAWINGS FIG. 1 is a side elevation of an apparatus for carrying out theinvention;

FIG. 2 is a cross-sectional view taken along line 2-2 1 of FIG. I; and,

FIG. 3 is a cross-sectional view taken along line 3-3 of FlG. l.

Referring to the drawings, indicated at 10 is a pipeline or conduitforming a portion ofa pneumatic system through which wood chips arebeing conveyed. Conventionally, such pipelines are circular incross-section being on average 6 to 24 inches in diameter, and woodchips are conventionally carried in an air stream having a velocity ofabout 6,000 feet per minute. ln such con veying systems the wood chipsare evenly dispersed in the conveying air stream as opposed to beinglayered as in belt conveyors and the like. In accordance with theillustrated embodiment of the present invention, the conveying system isprovided with a conduit section 12 that is essentially of trapezoidalcross-section but which has a cross-sectional area substantially equalto that of the pipeline 10. A suitable transition 14 is provided ateither end of the section 12 so as to permit its connection within theline 10. For purposes to be made apparent, section 12 preferably has alength of 6 to 12 feet.

Means are provided to effect exposure of the wood chipsin the section 12to high energy electrons. The illustrated means comprises a suitablesource of high energy electrons l8 capable of producing a narrow beam ofhigh speed electrons, the energy of which should be in the order ofseveral hundred thousand volts. The generator may be of the VAN de GRAFFtype or any other suitable type.

The electron beam generated by the source 18 in the scanner systemillustrated is projected downwardly into a scanner 20 which is providedwith means to cause the beam to reciprocate in a linear path having itsaxis coincident to the axis of the conduit section 12. The cyclingfrequency of the electron beam is preferably several hundreds tothousands of cycles per second. The deflection of the beam may beobtained by any conventional means such as by magnetic means or byelectrostatically operating deflecting means, all of which are wellknown to the art. The scanner 20 comprises an evacuated chamber definedby vertical parallel sidewalls 24, 26 and inclined end walls 28, 30. Thebottom wall 32 of the scanner is preferably formed of titanium or othermaterial providing low resistance to the passage of electronstherethrough but resistant to temperature effects caused by the electronbombardment. Likewise, the top wall 34 of the pipe section 12 is formedof titanium or other easily electron penetrable material. The spacebetweenwalls 32, 34 is washed with air in order to provide cooling ofwall 32. The double effect of walls 32, 34 and the intervening spaceprovide additional protection to the evacuated area of the scanner 20.An alarm or shut-down device (not shown) may be provided to indicate anyrupture in the wall 34, which will be indicated by increase in pressurein the chamber 36, and to prevent damage to wall 32 and con taminationof the scanner chamber.

Preferably a suitable screen is provided beneath the wall 34 to protectsuch wall from chip impingement and air may be injected into the spacebetween such screen and wall 34 by nozzles positioned along conduitsection 12 to wash the screen of chips and provide additional cooling towall 34.

In place of a scanner type beam source any other suitable electronsource may be utilized which will provide a flux of electronstransversely of the conduit section 12 so as to enable complete exposureof chip surfaces at sufficiently high dosages. For example, a sourcewhich will provide a steady or pulsed curtain of electrons extending thelength of the conduit section 12 may be utilized.

After entrance into the conduit the electron beam of whichever type usedwill spread latterly by reason of the repulsive force between electronsand the scattering resulting from impact with air molecules to form adiverging beam. The trapezoidal configuration of the pipe section 12 isselected so that the side walls 38, 40 are substantially coincident withthe effective side edges ofthe electron beam so as to obtainsubstantially uniform coverage across the entire width of the conduitsection 12. As the wood chips pass along the conduit section 12, due tothe magnus effect on the flat chip surfaces they will continually tumbleand change elevation across the pipe section. In a six-foot length oftravel the wood chip will ordinarily rotate and change elevation aboutsix times when conveyed in an air stream having a velocity of about6,000 ft. per minute. As will be apparent at the high oscillationfrequency of a scanned electron beam a chip will be exposed to the beama multitude of times during the course of travel through the section 12to assure that all surfaces of the chip will be exposed to electronbombardment. The radiation intensity is controlled such that with thevelocity and attitude variations of the chips, the chips accumulatesufficient irradiation over all surfaces to attain the desiredsterilization or other effect, which in most instances is thought to bebetween 0.1 and 0.5 megarads.

The energy level ofirradiation required for treatment of wood chips inaccordance with the invention necessitates that the irradiation beapplied over a relatively large area. One requirement is imposed bylimitations of the window through which the irradiation must pass. Theenergy must be spread out over a large area of the window to preventoverheating. More importantly the irradiation must be applied over anelongated area in order to permit the wood chip which is beingirradiated to sufficiently change in attitude and position so as toexpose all surfaces of the chip to the desired amount of irradiation asit travels through the conduit section. Thus, as a practical matter theirradiation must be supplied to the conduit in an area of applicationthat is rectangular in configuration when viewed in a direction normalto the axis of the conduit. The length of application should be aminimum of about six feet to permit complete exposure of all surfaces ofthe chip where complete exposure of chips is desired on all surfaces.Experience has indicated that a length of travel of 12 feet is adequatewhere the air velocity is 100 feet per second in a conduit sectionhaving a cross-sectional area slightly greater than 1 square foot. Ifanother type of source is utilized, the source intensity and conveyingconditions are adjusted so as to attain the desired uniform exposurelevel over the entire chip.

In order to obtain the required dosage on all surfaces of a chip withoutdisrupting the'pneumatic conveying characteristics of a system orreducing the conveying air velocity, it is necessary to convey the chipsthrough an irradiating conduit section which does not depart too much inconfiguration from that of the conveying pipe.

The pipe section 12 is preferably surrounded by a housing 44 defining achamber 46 through which water or other coolant fluid may be circulatedto prevent overheating of the section 12 as a result of the electronbombardment.

Because of the fact that essentially all surfaces of the chips areexposed to radiation by their dispersion and tumbling in their passagethrough the section 12, the source of electrons 18 can be of much lesserintensity than if attempts were made to treat the chips by bombardmentonly from one side, such as by treating them on a belt conveyor or thelike. In the latter case the intensity of the source would have to besuch as to permit penetration of the layered chips by the electrons.

lt has been found that treatment of chips by electron bombardmentsubstantially increases the storage life of the chips or, putting itconversely, the deterioration of chips over a given length of time ismaterially reduced.

Perhaps more importantly, it has been discovered that irradiation ofwood chips improves the pulp obtained from the chips in quality andyield. It has been found, for example, that the pulp freeness issubstantially increased but that the strength of the pulp is maintainedor increased. It has also been found that the lignin content of the pulpmay be higher. This can reduce the necessary pulping time and, ofcourse, reduces the amount of lignin that has to be disposed of, whileat the same time increasing the strength properties and the yield ofpulp obtained from a given amount of wood chips.

EXAMPLE The effectiveness of a system such as has been described hereinfor substantially achieving uniform irradiation of wood chips is shownin the following example.

A pneumatic conveying system having a fourteen inch diameter pipesupplied with air at a rate to cause air to pass through the system atfeet per second and convey wood chips therethrough at a velocity ofabout eighty feet per second was utilized. Mill run Douglas fir chipswere conveyed through such a system and through a pipe section having atrapezoidal crosssection similar to that shown in FIG. 3 of theapplication and of 12 feet in length. An electron source was providedfor irradiating wood chips passing through such section, the sourcehaving a potential of 500 kilovolts and a beam current of 200 milliampswhich was swept along the length of the pipe section at lOO cycles persecond. To measure the dosimetry amberperspex dosimeters were insertedin the pipeline at a point forward of the irradiation section so as topass therethrough together with the wood chips which were being conveyedin the line. These dosimeters were Y4 X X /8 inch or approximately thesize of wood chips although they were of slightly greater density. Afterpass ing through the equipment, the dosimeters were collected and thedosage received by each dosimeter analyzed and recorded. The dosagedistribution is shown in the table below.

TABLE Dosage Number (Megarads) Dosimetcrs 0 1 .05 0 .10 3 .l l 0 .l2 8.l3 7 .14 23 .15 24 .16 I3 .17 4 .l8 1 .l9 2

.20 0 2| 3 22 O .23 l .24 2 .31 l .34 l

If chips were to be conveyed through a line with relatively uniformdistribution across the crosssection, but without the tumbling andmigration that is a part of the process of the present invention, thedistribution of dosage would not be substantially uniform around themedium level in the manner obtained herein but a majority of theparticles would receive little (i.e. less than 0.1 Megarads dosage) orno irradiation, while only a small number would acquire irradiation atmedium and high levels because of the isodose profile that occurs withinthe pipeline relative to its cross section. A relatively high energylevel of irradiation is present at the entrance of the beam to thepipeline, but the energy level rapidly falls off as the beam spreads outwithin the pipeline:

Having illustrated and described a preferred embodiment of the inventionit should be apparent to those skilled in the art that it permits ofmodification in arrangement and detail.

I claim:

1. Apparatus for treating wood chips with electrons comprising apneumatic conveying system for conveying said wood chips including aconduit of substantially uniform cross-section and means for propellingair and wood chips therethrough, at a velocity such as to cause chips totumble and change elevation in said conduit as they are carriedtherethrough,

and means for injecting high energy electrons into a straight section ofsaid conduit free of obstruction over a substantially length sufficientto enable chips to tumble and migrate substantially through-.

out the. cross-section of said conduit section whereby all surfaces ofsaid chips will be exposed to irradiation by said electrons andaccumulate the required dosage from the various energy levels of theinjected electrons in said conduit substantially uniformly over all thechips and over each chip.

2. Apparatus for treating wood chips with electrons comprising:

means for creating a beam of high energy electrons,

cycling means for causing said beam to reciprocate in a linear path,

a pneumatic conveying system for conveying said wood chips including aconduit section free of obstruction having its axis coincident to saidlinear path and an electron transparent window in said path and meansfor propelling air and wood chips through said conduit section at avelocity such as to cause chips to tumble and migrate throughout thecross-section of said conduit section so that all surfaces of said chipswill be substantially uni formly exposed to said electron beam.

3. Apparatus as set forth in claim 2 wherein said conduit section hasside walls substantially coincident with the effective side edges ofsaid beam.

4. Apparatus as set forth in claim 2 wherein said conduit section istrapezoidal in cross-section.

5. Apparatus as set forth in claim 4 wherein said conduit section is ofuniform cross-sectional area throughout its length, and said pneumatic:system comprises a conduit means of cross-sectional area substantiallyequal to the area of said conduit section connected to the inlet end ofsaid section whereby the chips maintain a substantial uniform velocitythrough said section.

6'. The apparatus of claim 2 wherein said conduit section is of a lengthof about 12 feet and said cycling means is adapted to cause said beam tosweep through substantially the entire length of said conduit section.

7. ln apparatus for treating wood chips with electrons the combinationcomprising:

means defining an evacuated chamber,

means for creating a beam of high energy electrons at one end of saidchamber and directing said beam toward the opposite end,

means to cause said beam to reciprocate in a linear path at saidopposite end,

electron permeable window means in said opposite end of said chamber topermit emergence of electrons therefrom, a conduit section adjacent saidchamber opposite end having its axis coincident with the axis of saidpath,

said conduit section being trapezoidal in cross section with the sidewalls of said section substantially coincident with the effective sideedges of said electron beam,

the top wall of said conduit section comprising electron permeablewindow means so as to permit entry of said beam into said conduitsection,

and means for pneumatically conveying wood chips through said conduitsection at a high velocity whereby the wood chips will tumble andmigrate throughout the cross-section of said conduit section exposingall surfaces of said chips to bombardment by said electrons and toelectrons of various energy levels whereby said chips will receive substantially uniform dosage.

=l =l =l k

1. Apparatus for treating wood chips with electrons comprising apneumatic conveying system for conveying said wood chips including aconduit of substantially uniform cross-section and means for propellingair and wood chips therethrough, at a velocity such as to cause chips totumble and change elevation in said conduit as they are carriedtherethrough, and means for injecting high energy electrons into astraight section of said conduit free of obstruction over asubstantially length sufficient to enable chips to tumble and migratesubstantially throughout the cross-section of said conduit sectionwhereby all surfaces of said chips will be exposed to irradiation bysaid electrons and accumulate the required dosage from the variousenergy levels of the injected electrons in said conduit substantiallyuniformly over all the chips and over each chip.
 2. Apparatus fortreating wood chips with electrons comprising: means for creating a beamof high eneRgy electrons, cycling means for causing said beam toreciprocate in a linear path, a pneumatic conveying system for conveyingsaid wood chips including a conduit section free of obstruction havingits axis coincident to said linear path and an electron transparentwindow in said path and means for propelling air and wood chips throughsaid conduit section at a velocity such as to cause chips to tumble andmigrate throughout the cross-section of said conduit section so that allsurfaces of said chips will be substantially uniformly exposed to saidelectron beam.
 3. Apparatus as set forth in claim 2 wherein said conduitsection has side walls substantially coincident with the effective sideedges of said beam.
 4. Apparatus as set forth in claim 2 wherein saidconduit section is trapezoidal in cross-section.
 5. Apparatus as setforth in claim 4 wherein said conduit section is of uniformcross-sectional area throughout its length, and said pneumatic systemcomprises a conduit means of cross-sectional area substantially equal tothe area of said conduit section connected to the inlet end of saidsection whereby the chips maintain a substantial uniform velocitythrough said section.
 6. The apparatus of claim 2 wherein said conduitsection is of a length of about 12 feet and said cycling means isadapted to cause said beam to sweep through substantially the entirelength of said conduit section.
 7. In apparatus for treating wood chipswith electrons the combination comprising: means defining an evacuatedchamber, means for creating a beam of high energy electrons at one endof said chamber and directing said beam toward the opposite end, meansto cause said beam to reciprocate in a linear path at said opposite end,electron permeable window means in said opposite end of said chamber topermit emergence of electrons therefrom, a conduit section adjacent saidchamber opposite end having its axis coincident with the axis of saidpath, said conduit section being trapezoidal in cross-section with theside walls of said section substantially coincident with the effectiveside edges of said electron beam, the top wall of said conduit sectioncomprising electron permeable window means so as to permit entry of saidbeam into said conduit section, and means for pneumatically conveyingwood chips through said conduit section at a high velocity whereby thewood chips will tumble and migrate throughout the cross-section of saidconduit section exposing all surfaces of said chips to bombardment bysaid electrons and to electrons of various energy levels whereby saidchips will receive substantially uniform dosage.